Is it possible (plugin?) to use a camera input (solidview or materialview) in the compositor as a preview?
If you're using 2.8x, you can change the render engine to 'Workbench.'

It's the equivalent of a viewport render in previous Blender versions, except it ports into Blender's rendering system, and therefore the compositor.  It should render a frame about as quickly as the viewport, and gives you to extra options (depth of field, outline, basic shadows, etc.)